Questions tagged [cryptography]

Questions about the construction and analysis of protocols and algorithms for secure computation and communication (including authentication, integrity, and privacy aspects).

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Solving subset-sum encryption (Princeton Creative Assignment)

The question: https://www.cs.princeton.edu/courses/archive/spring03/cs226/assignments/password.html Input files: ftp://ftp.cs.princeton.edu/pub/cs226/password/ The question asks to use a symbol table ...
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Manually Performing ECB & CBC

I am learning about block ciphers and while I understand the concept of Electronic Codeblock mode and cipher block chaining, I could not find any relevant practical examples online. Can someone ...
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2answers
63 views

Undetectable error correcting codes

I have a 256 bit string (indistinguishable from random) which I wish to encode into a greater length string using an error correction code. The result must also be indistinguishable from random. It ...
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Is this Correct, the existence of cryptography requires $UP \cap Co-UP \not\subseteq BPP$

Is this Correct, the existence of cryptography requires $UP \cap Co-UP \not\subseteq BPP$? Or does it require $UP \not\subseteq BPP$?
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Books for learning Cryptocurrency

Please suggest a good for mathematical section of studying cryptocurrency.
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Are there any quantum resistant equivalents to RSA?

Are there any quantum resistant equivalents to RSA? Basically are there any methods of cyrtopgraphy that hame the same user as RSA but are resitant to quantum computers.
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Asking for your help with LFSR, linear automaton

I am a software developer new to the site. I am currently learning about LFSR (linear feedback shift register). Every day I solve a question which is given to me, but today I am lost. I can not solve ...
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1answer
26 views

One-way function is not injective when it is in NP

Let us $\Sigma = \{0,1\}$ and $f: \Sigma^* \rightarrow \Sigma^* \in FP$ for which is valid that $\exists k: \forall x \in \Sigma^* : \lvert x \rvert ^ {1/k} \leq \lvert f(x) \rvert \leq \lvert x \...
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How to perform matrix multiplication in Mixing Columns step of AES?

I am studying AES and trying to implement it. But I am having difficulty understanding the Mixing Column step. In that step we have to perform matrix multiplication between the state matrix and ...
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27 views

Accessible CS Math Job [closed]

I am a CS undergrad and a huge enthusiast of pure math. I have been doing competitive programming and proofs for a while. My ambition is to become an academic in theoretical computer science. The ...
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1answer
35 views

Underlying codes for Niederreiter cryptosystems

Niederreiter cryptosystem is usually described by a parity check matrix $H$ over $\mathbb{F}_{2^n}$. The minimum distance $d$ is given by $d := min\lbrace k \text{ such that there are $k$ linearly ...
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Decryption of RSA

How can we decrypt an RSA message if we only have the public key? For example, Message: 21556870,12228498 Public Key: ...
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Converting Smart Contracts into Arithmetic Circuits [closed]

Are there any existing programs available to convert smart contracts into Arithmetic Circuits with which I can implement and experiment with Secure Multiparty Computation (MPC) protocols?
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Absolute limit in Huffman encoding

I have computed the average code length using the Huffman encoding. Now, I am asked to compare the average code length with Absolute limit but I ain't sure about the absolute limit here is :( Could ...
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Extracting text from a hidden image

Say i have cropped an image and have hidden some part of it, like shown below Is it possible for anyone to download this image and able to decipher what the hidden text is?
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Digitally signing a digital signature

I have the following question in a list of security exercises. What it can be the sense of digitally signing a digital signature? Sincerely I can't think of anything. Someone could answer?
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Abusing ElGamal in order to attack a known encrypted text

I saw a very interesting question regarding Elgamal cryptosystem that I don't know its answer. It is really interesting and I would be very happy if you could elaborate on it and explain the tricky ...
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learning the private key by reusing same random variable k in ELgamal

i wonder: if for some reason, someone, say alice, sends unencrypted messages to bob and signs it using elgamal signature, can oscar,the adversary, gain knowledge of the private key if alice reused the ...
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equivalent sub-keys in DES encryption

i am trying to understand the DES cryptosystem and was wondering: what would've happened if all of the sub-keys were equal? does it reduce the security? can we actually find the keys if we know all ...
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1answer
30 views

Period of sum of two periodic sequences

I was wondering, what is to be the shortest possible key using Vigenere encryption, if a text is ciphered one time with a key of length $i$ using Vigenere and second time with a key of length $j$ ...
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1answer
35 views

Determining key from plaintext and ciphertext (Vigenère)

I was wondering why mathematically we can know the key in the Vigenère cipher if we know in advance a message and its encryption.
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Can a public key be used to decrypt a message encrypted by the corresponding private key?

From what I have seen about usage of a pair of public and private keys, the public key is used for encrypting a message, and the private key is used for decrypting the encrypted message. If a message ...
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34 views

Is 'the bombe' technically a computer?what is technically meant to be a computer?

Some say that 'the bombe' created by Alan Turing is technically not a computer despite decrypting the codes. Why is it so? Is 'the bombe' technically a computer? First of all what is technically ...
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Solve SUBSET SUM for Reciprocals of Primes

Let $p_1, ..., p_n$ distinct prime numbers with $P = \prod_{i=1}^{n}{p_i}$ and $A=(a_1, ..., a_n)$ with $a_i = P/p_i$. Problem Show the SUBSET SUM problem $(A, \alpha)$ can be solved in polynomial (...
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If current time in milliseconds is considered good enough random seed for a pseudorandom number generator, why not just use that time directly?

I was reading about pseudorandom number generators and how they need a seed and how that seed is usually current system time in milliseconds. One of the most common algorithms is the linear ...
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1answer
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Cryptography: Play Fair cipher

In our university, we were taught that since all 26 letters cant be placed together in a $5*5$ square matrix we must put $i/j$ together like this represented in the below diagram. Here our key is $...
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Diffie-Hellman and its disadvantage with large primes

I was reading our university slide on the Diffie-Hellman where it is mentioned that one of the disadvantages of D-H is that For large prime, $p-1$ is an even number so, $\mathbb{Z}^*_p$ will have a ...
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Why hasn't there been an encryption algorithm that is based on the known NP-Hard problems?

Most of today's encryption, such as the RSA, relies on the integer factorization, which is not believed to be a NP-hard problem, but it belongs to BQP, which makes it vulnerable to quantum computers. ...
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2answers
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What are the elements of the modular ring mod 7? [closed]

Are the elements of a modular ring simply the set of all the numbers from 1 to p−1? in this case p−1 = 6 ? I asked this on the math stack exchange https://math.stackexchange.com/q/3375667 and was ...
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26 views

Weaknesses arising from using same key in both channel directions

I came across the following question: Which of the following risk may arise, when same key is used to encrypt both directions of a communication channel, that are not present if using different keys ...
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1answer
137 views

Understanding Incentive Compatibility of pooled Bitcoin Mining paper

I'm trying to understand the paper Incentive Compatibility of Bitcoin Mining Pool Reward Functions (Schrijvers, Bonneau, Doneh and Roughgarden, in Financial Cryptography and Data Security – ...
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1answer
69 views

Perfect Probabilistic Encryption still requires key length about as long as message

Let $(E,D)$ be a probabilistic encryption scheme with $n$-length keys (given a key $k$, we denote the corresponding encryption function by $E_k$) and $n+10$-length messages. Then, show that there ...
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RSA cryptosystem: Encrypting a signed message

I've started to read the section about the RSA cryptosystem in CLRS (page 958) and I don't understand the way it describes how to encrypt a signed message. If Bob wants to send a message $M$ to Alice:...
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Is time complexity more important than space complexity?

I've noticed quite a few cryptographic algorithms speak mainly of the time complexity of an algorithm. For example, with a hashing function h, find x given y = h(x). We normally speak on how long it ...
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A tree-like data structure with rights delegation for distributed computing

Every actor can create a root node and delegate a right to add a child node. Every node contains name of its’ creator or who added it, and value S. Sum of all values S at the same level of the tree ...
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1answer
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Is F(F(s,x), x) necessarily a PRF if F is?

Given a PRF - F, is $G_s(x)=F_{F_s(x)}(x)$ necessarily a PRF? First I thought how to tackle this problem. First I tried using a hybrid argument: $|\mathbb P[F_{F_s(x)}(x)] -\mathbb P[f(x)]|\leq |\...
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Inverting a function [closed]

I posted this question on mathoverflow two years ago but got no answer: Let $w = a_0 \cdot a_1 \cdots a_{n-1} $ be a word from $ \{0,1\}^n $, $|w| = n$ Let $m = \sum_{i=0}^{n-1}{ a_i \cdot 2 ^ {n-1-...
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2answers
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Why don't most websites I visit seem to use TLS with Ephemeral Diffie-Hellman?

When I click "view certificate," under Public Key Algorithm it usually says "RSA Encryption" or "Elliptic Curve Public Key." I assume this is the algorithm used to agree on a premaster secret (...
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1answer
25 views

Why does Feistel encryption algorithm encode half block every time?

Why does Feistel encryption algorithm encode half block every time? What would happen if the entire block is encrypted in each step?
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643 views

Is it possible to create a “Time Capsule” using encryption?

I want to create a digital time capsule which will remain unreadable for some period of time and then become readable. I do not want to rely on any outside service to, for instance, keep the key ...
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1answer
59 views

Anonymous Visibility Check in P2P networks

I'm working on a problem for a P2P network for games. The problem is the following: Consider two opponents on a grid, each stores it's own position. Player 1 wants to know if it sees player 2. In ...
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2answers
825 views

Can you prevent a man in the middle from reading the message?

I have heard about all these Man-In-The-Middle Attack preventions and I am wondering, how this can possibly work if the man in the middle only listens to your stream and does not want to change the ...
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What is the complete version of the paper: “How to Generate and Exchange Secrets (extended abstract)” by Andrew Yao?

I've found numerous places that claim that the paper "How to Generate and Exchange Secrets" by Andrew Yao introduces garbled circuits as a solution to the secure multiparty computation problem. ...
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Finding small-length RSA private key

I was wondering if there is an algorithm to derive $p$ and $q$ or is it simply trial and error? Consider the following RSA crypto-system with public key $(437,13)$. Since the numbers are so small, it ...
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1answer
95 views

fully homomorphic encryption with information-theoretic security?

An encryption algorithm with information-theoretic security is one which even with infinite amount of computation cannot be broken. That is, given only the ciphertext, no amount of computation can ...
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Is it possible to build a secure PRG from two functions one of them being a PRG?

Having two deterministic functions $G_1, G_2 : \left\{0,1\right\}^\lambda \rightarrow \left\{0,1\right\}^{\lambda+l}$, at least one of which is a secure PRG. Being $\alpha$ a constant, is it possible ...
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203 views

Is the bitwise-xor of a Uniform bit string and a non_uniform bit string Uniform?

Having two bit strings $x,y \in \left\{0,1\right\}^n$, where $x$ is selected following a uniform distribution but $y$ is not. Is $z = x \oplus y$ uniform?
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1answer
41 views

DES encryption - what happens if all sub-keys are the same?

What will happen if we change the DES encryrption algorithm to use the same key in each round ? How will it affect the encryption - will it only make the encryption same as decryption or are the more ...
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1answer
112 views

Prove hash family is 3-wise independent

Let $q$ be a prime number and let $\mathbb{Z}_q = \left\{1,\dots,q-1\right\}$; I need to prove that the family $\mathcal{H} = \left\{h_s \colon \mathbb{Z}_q \rightarrow \mathbb{Z}_q\right\}_{s \in \...
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1answer
48 views

Can I decrypt an RSA algorithm using only the message I am wanting to decrypt, an “e” value and a “n” value?

I have a ciphertext $c$ encrypted with RSA algorithm that needs to decrypt. I have the public key $(n,e)$. Is it possible to decrypt with this amount of information?

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